The present invention relates to a method for preparing a pharmacologically active cyanoguanidine derivative and, in particular, to a method for preparing N-cyano-N'-methyl-N"-[2-{(4-methyl-5-imidazolyl)-methylthio}-ethyl] guanidine which is represented by the following formula: ##STR1##
The compound with which the present invention is concerned is generally referred to as "cimetidine" and is pharmacologically useful in that the compound has an inhibiting action to the histamine-H.sub.2 -receptor.
Various methods for the preparation of the compound of the formula (I) above have heretofore been proposed such as disclosed in, for example, Japanese Laid-open Patent Publication No. 49-75574 (corresponding to United Kingdom Patent Applications No. 41160/1972 and No. 6154/1973), laid open to public inspection in 1974, and Japanese Laid-open Patent Publication No. 51-125074 (corresponding to United Kingdom Patent Application No. 38257/1974) laid open to public inspection in 1976. According to these publications, the prior art methods are complicated and time-consuming in that they require a plurality of reaction processes to be performed with the consequent repetition of isolation and purification of the respective intermediate reaction products. More specifically, each intermediate reaction product resulting from one reaction process is required to be isolated or purified before it is subjected to the subsequent reaction process. Recovery of the final intended product involves the employment of a column chromatography to effect the isolation and/or purification. Not only is the column chromatography expensive and costly, but the employment of the column chromatography requires the subsequent condensation of the diluted solution obtained therefrom. In addition, the known methods give a relatively low yield of approximately 30%.